Carbon nanotubes are cylindrical carbon molecules with novel properties that make them useful in a wide variety of applications (e.g., nano-electronics, optics, materials applications, etc.). CNTs exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat.
A CNT is cylindrical, with at least one end typically capped. The CNT name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers (approximately 50,000 times smaller than the width of a human hair), while they can be up to several centimeters in length. There are two main types of nanotubes, single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).
Nanotubes are composed entirely of sp2 bonds, similar to those of graphite. This bonding structure, stronger than the sp3 bonds found in diamond, provides the molecules with their unique strength.
Most SWNTs have a diameter of close to one nanometer, with a tube length that can be many thousands of times larger. SWNTs with length up to orders of centimeters have been produced. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite (called graphene) into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (n,m) called the chiral vector. SWNTs are a very important variety of carbon nanotube because they exhibit important electric properties that are not shared by the multi-walled carbon nanotube (MWNT) variants. Multiwalled nanotubes (MWNT) consist of multiple layers of graphite rolled in on themselves to form a tube shape. There are two models that can be used to describe the structures of multiwalled nanotubes. In the Russian Doll model, the nanotubes are arranged in concentric cylinders, e.g., a SWNT within a larger SWNT. In the Parchment model, a nanotube is rolled in around itself, resembling a scroll of parchment or a rolled up newspaper.
Carbon nanotubes are one of the strongest materials known to man, both in terms of tensile strength and elastic modulus. This strength results from the covalent sp2 bonds formed between the individual carbon atoms. A SWNT can have a tensile strength of 63 GPa. In comparison, high-carbon steel has a tensile strength of approximately 1.2 GPa. CNTs also have a very high elastic modulus, in the order of 1 TPa. Since carbon nanotubes have relatively low density, the strength to weight ratio is therefore truly exceptional.
Carbon nanotubes have already been used as fibers in polymers and concrete to improve the mechanical, thermal and electrical properties of the bulk composite product. Researchers have also found that adding them to polyethylene increases the polymer's elastic modulus by 30%. In concrete, they increase the tensile strength, and halt crack propagation.
Advanced composite systems comprising multiple plies laminated together in various orientations provide a type of material that has proven useful in several types of applications. In such multi-ply composite systems, a naturally occurring toughening mechanism known as large-scale bridging phenomena takes place at the ply interface. In cases where the ply orientation is very similar, such as the interface between two 0° plies, fibers are known to “bridge” cracks as they advance increasing the toughness of interface, e.g., in an aerospace graphite/epoxy composite system, fiber bridging of this sort has been measured to increase toughness 100-200×. This significant increase in toughness is achieved simply though fiber waviness bridging the interface region.